1. Field of the Invention
The present invention concerns a free-cutting steel. More particularly, it concerns a low carbon calciumsulfur containing free-cutting steel exhibiting improved machinability prepared by regulating the Brinell hardness of the steel matrix to a selected suitable range and by forming and maintaining a suitable amount of oxide inclusions of type JIS-A2 (ASTM-C) which soften or fuse at a temperature not higher than 1400.degree.C in the steel. Drawbacks of conventional killed low-carbon manganese sulfide containing free-cutting steel are remedied in the present steel.
2. Description of the Prior Art
It has been common to use low-carbon sulfur containing free-cutting steel exhibiting improved machinability, in which soft-sulfide inclusions in the form of MnS are dispersed. Tool service life is extended when the steel is cut with a high speed steel tool, because the sulfide inclusions bring about effects such as promotion of strain by concentrated stress in the contact area of the tool and the cut material, or crack propagation, decrease of tool wear through internal lubrication, and prevention of built up edge formation. However, if there exist in the steel oxide or carbonitride inclusions, e.g. SiO.sub.2, Al.sub.2 O.sub.3, TiO, Ti(CN), having hardness higher than that of the tool material among the inclusions which are inevitably contained in the steel due to formation of deoxidation products or addition of alloying elements during deoxidation treatment of molter steel, these inclusions act like fine abrasive grains to abrade and damage the tool resulting in decrease of the tool service life.
Thus, low-carbon sulfur containing free-cutting steels generally made nowadays are so-called Mn-S killed steels which have not been subjected to a strong deoxidation so as to keep the content of the hard inclusion low. In the Mn-S killed low-carbon sulfur containing free-cutting steel, bubble formation in the surface layer of the ingot is unavoidable because generation of carbon monoxide gas during solidification of the ingot, even though rimming action and the generation of CO gas are surpressed at higher contents of manganese and sulfur. The bubbles, which are defects of the billet surface further increase billet surface conditioning time and cause decrease in yield. Moreover, insufficient deoxidation results in serious sulfur segregation, and hence, machinability and mechanical properties vary significantly from ingot to ingot.